In the field of modern poultry farming, prevention of diseases by vaccination is a major means for sanitation regardless of the kind of chicks, being raised i.e. a chick for breeding, a chick for laying eggs or a chick for meat. The vaccination, however, has to be done so frequently that personnel expenses become much higher, causing an economical disadvantage for a poultry farmer. In order to avoid this disadvantage, one can contemplate simply mixing several known vaccines. However, there is a problem of interference occuring between viruses in the case of a mixture of live vaccines and there is also a limitation in the mixing amount in the case of a mixture of inactivated vaccines. In addition, in the case of a mixture of a live vaccine and an inactivated vaccine, a titer decrease is observed due to an adsorption of a live vaccine antigen to a gel (adjuvant).
Recently, taking into account the above situations, alternative method has been attempted to employ a virus vector, i.e. multiple genes of vaccine antigenes are incorporated into a single virus to prepare a multivalent live vaccine. This method makes it possible to prepare a multivalent live vaccine without causing the interference between viruses or the increase of inoculation amount in case of the mixture of inactivated vaccines as mentioned above.
Hitherto, a research has already been conducted to use a virus as a vector in various viruses such as vaccinia virus, adenovirus, herpes simplex virus, retrovirus, and the like, and HBs antigen (Hepatitis B surface antigen) or glycoproteins of rabies virus or herpes zoster virus have successfully been expressed in vitro. However, these viruses other than vaccinia virus are a virus having an oncogencity and hence the administration of these viruses to human or animals is restricted and not practical from the viewpoint of safety. As to vaccinia virus, although the virus itself is safe, it cannot be used effectively as a virus vector for birds, at which the present invention is aimed, since the birds to be inoculated are not an original host of the vaccinia virus. For the same reason, the other viruses as mentioned above cannot be used effectively as a virus vector for birds.
Use of avian poxvirus (e.g. chick fowlpox virus) as a vector also has been suggested and the virus has already been studied for use as a virus vector. It is reported that an exogenous gene can be incorporated into the virus DNA [Saeki et al., Abstract of the 35th Meeting of Japan Virology Society, page 209 (1987)].
However, in the modern poultry field, immunity against fowlpox lasts for only short period of time, and hence, several inoculations of a vaccine virus (attenuated fowlpox virus or ornithosis virus) are usually required during the breeding of chick. Consequently, when the poxvirus is used as the virus vector, a frequent vaccination is still required even though a virus vector wherein plural antigens are incorporated is prepared and used as a vaccine.